The invention relates to a paper transport device for feeding single sheets of paper on continuous forms.
Paper transport devices of this kind have already been suggested and are being used successfully.
For instance, DE-PS No. 31 49 667 describes an attachable transport device for margin-perforated record carriers which is attached to a printer normally intended for single sheets and which contains a tractor drive. Both the single sheets and the continuous paper, if needed with transport device attached, are conducted around the platen by one common paper channel. The tractor drive is designed so that the continuous paper is pulled across the platen. In the printers mentioned above, the continuous paper as well as the single sheets are guided around the platen in the paper guide channel by paper bail rollers. The various rollers are engaged in both continuous paper and single sheet operation.
In addition, it is common practice for the feeding of single sheets to use automatic "feeders". Feeders include cassettes containing a paper supply, and precede the transport device for continuous paper in the printer. The paper to be printed on is automatically taken from the feeder as needed and fed to the printer or platen.
To make both removal of single sheets from the feeder and precise re-depositing of the paper reliable in such automatic operation, the contact pressure of the paper bail rollers must be relatively great, so that no offsetting between record carrier and platen will occur.
On the other hand, if continuous paper is used, a tractor drive has already been suggested in which the paper tractor is mounted upstream of the platen in the direction of paper transport, feeding the continuous paper by pushing it. The platen is essentially a mere guide for the continuous paper, and to make the paper pass through cleanly, the circumferential speed of the platen is set slightly greater than the tractor's feed rate.
This type of feed is inappropriate for feeding single sheets because in single sheet operation the paper feed rate depends essentially on the circumferential speed of the platen. This is disadvantageous particularly when graphics are to be printed, because the transition from continuous paper to single sheets leads to a distortion of the graphic representation.